Twist conformation for the cis N,N-dimethylphosphoramidate of thymidine 3′,5′-cyclic phosphate

PMR analysis assigns twist and chair predominant conformations to the Rp and Sp diastereomers, respectively ($\text{3}$$\text{a}$ and $\text{3}$$\text{b}$), of thymidine 3′,5′-cyclic N,N-dimethylphosphoramidate, $\text{3}$. The twist conformer population of $\text{3}$$\text{a}$ is about 64 (acetone) to 75% (toluene).